Module 4: Processes!
•  Process Concept!
•  Process Scheduling!
•  Operation on Processes!
•  Cooperating Processes!
•  Interprocess Communication!
Operating System Concepts!
4.1!
Silberschatz and Galvin ©1999
Process Concept!
•  An operating system executes a variety of programs:!
–  Batch system – jobs!
–  Time-shared systems – user programs or tasks!
•  Textbook uses the terms job and process almost
interchangeably.!
•  Process – a program in execution; process execution must
progress in sequential fashion.!
•  A process includes:!
–  program counter !
–  stack!
–  data section!
Operating System Concepts!
4.2!
Silberschatz and Galvin ©1999
Process State!
•  As a process executes, it changes state!
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–
–
–
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new: The process is being created.!
running: Instructions are being executed.!
waiting: The process is waiting for some event to occur.!
ready: The process is waiting to be assigned to a process.!
terminated: The process has finished execution.!
Operating System Concepts!
4.3!
Silberschatz and Galvin ©1999
Diagram of Process State!
Operating System Concepts!
4.4!
Silberschatz and Galvin ©1999
Process Control Block (PCB)!
Information associated with each process.!
•  Process state!
•  Program counter!
•  CPU registers!
•  CPU scheduling information!
•  Memory-management information!
•  Accounting information!
•  I/O status information!
Operating System Concepts!
4.5!
Silberschatz and Galvin ©1999
Process Control Block (PCB)!
Operating System Concepts!
4.6!
Silberschatz and Galvin ©1999
CPU Switch From Process to Process!
Operating System Concepts!
4.7!
Silberschatz and Galvin ©1999
Process Scheduling Queues!
•  Job queue – set of all processes in the system.!
•  Ready queue – set of all processes residing in main memory,
ready and waiting to execute.!
•  Device queues – set of processes waiting for an I/O device.!
•  Process migration between the various queues.!
Operating System Concepts!
4.8!
Silberschatz and Galvin ©1999
Ready Queue And Various I/O Device Queues!
Operating System Concepts!
4.9!
Silberschatz and Galvin ©1999
Representation of Process Scheduling!
Operating System Concepts!
4.10!
Silberschatz and Galvin ©1999
Schedulers!
•  Long-term scheduler (or job scheduler) – selects which
processes should be brought into the ready queue.!
•  Short-term scheduler (or CPU scheduler) – selects which
process should be executed next and allocates CPU.!
Operating System Concepts!
4.11!
Silberschatz and Galvin ©1999
Addition of Medium Term Scheduling!
Operating System Concepts!
4.12!
Silberschatz and Galvin ©1999
Schedulers (Cont.)!
•  Short-term scheduler is invoked very frequently (milliseconds)
⇒ (must be fast).!
•  Long-term scheduler is invoked very infrequently (seconds,
minutes) ⇒ (may be slow).!
•  The long-term scheduler controls the degree of
multiprogramming.!
•  Processes can be described as either:!
–  I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts.!
–  CPU-bound process – spends more time doing
computations; few very long CPU bursts.!
Operating System Concepts!
4.13!
Silberschatz and Galvin ©1999
Context Switch!
•  When CPU switches to another process, the system must save
the state of the old process and load the saved state for the new
process.!
•  Context-switch time is overhead; the system does no useful work
while switching.!
•  Time dependent on hardware support.!
Operating System Concepts!
4.14!
Silberschatz and Galvin ©1999
Process Creation!
•  Parent process creates children processes, which, in turn create
other processes, forming a tree of processes.!
•  Resource sharing!
–  Parent and children share all resources.!
–  Children share subset of parent’s resources.!
–  Parent and child share no resources.!
•  Execution!
–  Parent and children execute concurrently.!
–  Parent waits until children terminate.!
Operating System Concepts!
4.15!
Silberschatz and Galvin ©1999
Process Creation (Cont.)!
•  Address space!
–  Child duplicate of parent.!
–  Child has a program loaded into it.!
•  UNIX examples!
–  fork system call creates new process!
–  execve system call used after a fork to replace the
process’ memory space with a new program.!
Operating System Concepts!
4.16!
Silberschatz and Galvin ©1999
A Tree of Processes On A Typical UNIX System!
Operating System Concepts!
4.17!
Silberschatz and Galvin ©1999
Process Termination!
•  Process executes last statement and asks the operating system
to decide it (exit).!
–  Output data from child to parent (via wait).!
–  Process’ resources are deallocated by operating system.!
•  Parent may terminate execution of children processes (abort).!
–  Child has exceeded allocated resources.!
–  Task assigned to child is no longer required.!
–  Parent is exiting.!
  Operating system does not allow child to continue if its
parent terminates.!
  Cascading termination.!
Operating System Concepts!
4.18!
Silberschatz and Galvin ©1999
Cooperating Processes!
•  Independent process cannot affect or be affected by the
execution of another process.!
•  Cooperating process can affect or be affected by the execution of
another process!
•  Advantages of process cooperation!
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–
–
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Information sharing !
Computation speed-up!
Modularity!
Convenience!
Operating System Concepts!
4.19!
Silberschatz and Galvin ©1999
Producer-Consumer Problem!
•  Paradigm for cooperating processes, producer process produces
information that is consumed by a consumer process.!
–  unbounded-buffer places no practical limit on the size of the
buffer.!
–  bounded-buffer assumes that there is a fixed buffer size.!
Operating System Concepts!
4.20!
Silberschatz and Galvin ©1999
Bounded-Buffer – Shared-Memory Solution!
•  Shared data!
var n;!
type item = … ;!
var buffer. array [0..n–1] of item;!
in, out: 0..n–1;!
•  Producer process !
repeat!
…!
produce an item in nextp!
…!
while in+1 mod n = out do no-op;!
buffer [in] :=nextp;!
in :=in+1 mod n;!
until false;!
Operating System Concepts!
4.21!
Silberschatz and Galvin ©1999
Bounded-Buffer (Cont.)!
•  Consumer process !
repeat!
while in = out do no-op;!
nextc := buffer [out];!
out := out+1 mod n;!
!…!
consume the item in nextc!
! …!
until false;!
•  Solution is correct, but can only fill up n–1 buffer.!
!
Operating System Concepts!
4.22!
Silberschatz and Galvin ©1999
Threads!
•  A thread (or lightweight process) is a basic unit of CPU utilization;
it consists of:!
–  program counter!
–  register set !
–  stack space!
•  A thread shares with its peer threads its:!
–  code section!
–  data section!
–  operating-system resources!
collectively know as a task.!
•  A traditional or heavyweight process is equal to a task with one
thread!
Operating System Concepts!
4.23!
Silberschatz and Galvin ©1999
Multiple Threads within a Task!
Operating System Concepts!
4.24!
Silberschatz and Galvin ©1999
Example:Several Threads in Word Processor!
1- Interactive thread.!
2- Formatting thread in background.!
3- Auto-saving thread in background.!
Operating System Concepts!
4.25!
Silberschatz and Galvin ©1999
Example:Several Threads in Web Server!
- Threads in web server.!
Operating System Concepts!
4.26!
Silberschatz and Galvin ©1999
Example:Several Threads in Web Server (Cont.)!
- What happen if we have a web server with single thread? !
Operating System Concepts!
4.27!
Silberschatz and Galvin ©1999
Threads (Cont.)!
•  In a multiple threaded task, while one server thread is blocked
and waiting, a second thread in the same task can run.!
–  Cooperation of multiple threads in same job confers higher
throughput and improved performance.!
–  Applications that require sharing a common buffer (i.e.,
producer-consumer) benefit from thread utilization.!
•  Threads provide a mechanism that allows sequential processes
to make blocking system calls while also achieving parallelism.!
•  Kernel-supported threads (Mach and OS/2).!
•  User-level threads; supported above the kernel, via a set of
library calls at the user level.!
•  Hybrid approach implements both user-level and kernelsupported threads (Solaris 2).!
Operating System Concepts!
4.28!
Silberschatz and Galvin ©1999
Threads and Processes!
Operating System Concepts!
4.29!
Silberschatz and Galvin ©1999
Threads and Processes (Cont.)!
Operating System Concepts!
4.30!
Silberschatz and Galvin ©1999
Thread Resources!
Operating System Concepts!
4.31!
Silberschatz and Galvin ©1999
Threads’ States!
1- Spawn
2- Block
3- Unblock
4- Finish
Operating System Concepts!
4.32!
Silberschatz and Galvin ©1999
Threads’ States!
Operating System Concepts!
4.33!
Silberschatz and Galvin ©1999
User-level and Kernel-level Threads!
Operating System Concepts!
4.34!
Silberschatz and Galvin ©1999
User-level and Kernel-level Threads!
Operating System Concepts!
4.35!
Silberschatz and Galvin ©1999
User-level and Kernel-level Threads!
H.W.!
!
What is Jacketing?!
Operating System Concepts!
4.36!
Silberschatz and Galvin ©1999
Interprocess Communication (IPC)!
•  Mechanism for processes to communicate and to synchronize
their actions.!
•  Message system – processes communicate with each other
without resorting to shared variables.!
•  IPC facility provides two operations:!
–  send(message) – message size fixed or variable !
–  receive(message)!
•  If P and Q wish to communicate, they need to:!
–  establish a communication link between them!
–  exchange messages via send/receive!
•  Implementation of communication link!
–  physical (e.g., shared memory, hardware bus)!
–  logical (e.g., logical properties)!
Operating System Concepts!
4.37!
Silberschatz and Galvin ©1999
Implementation Questions!
•  How are links established?!
•  Can a link be associated with more than two processes?!
•  How many links can there be between every pair of
communicating processes?!
•  What is the capacity of a link?!
•  Is the size of a message that the link can accommodate fixed or
variable?!
•  Is a link unidirectional or bi-directional?!
Operating System Concepts!
4.38!
Silberschatz and Galvin ©1999
Direct Communication!
•  Processes must name each other explicitly:!
–  send (P, message) – send a message to process P!
–  receive(Q, message) – receive a message from process Q!
•  Properties of communication link!
–  Links are established automatically.!
–  A link is associated with exactly one pair of communicating
processes.!
–  Between each pair there exists exactly one link.!
–  The link may be unidirectional, but is usually bi-directional.!
Operating System Concepts!
4.39!
Silberschatz and Galvin ©1999
Indirect Communication!
•  Messages are directed and received from mailboxes (also
referred to as ports).!
–  Each mailbox has a unique id.!
–  Processes can communicate only if they share a mailbox.!
•  Properties of communication link!
–  Link established only if processes share a common mailbox!
–  A link may be associated with many processes.!
–  Each pair of processes may share several communication
links.!
–  Link may be unidirectional or bi-directional.!
•  Operations!
–  create a new mailbox!
–  send and receive messages through mailbox!
–  destroy a mailbox!
Operating System Concepts!
4.40!
Silberschatz and Galvin ©1999
Indirect Communication (Continued)!
•  Mailbox sharing!
–  P1, P2, and P3 share mailbox A.!
–  P1, sends; P2 and P3 receive.!
–  Who gets the message?!
•  Solutions!
–  Allow a link to be associated with at most two processes.!
–  Allow only one process at a time to execute a receive
operation.!
–  Allow the system to select arbitrarily the receiver. Sender is
notified who the receiver was.!
Operating System Concepts!
4.41!
Silberschatz and Galvin ©1999
Buffering!
•  Queue of messages attached to the link; implemented in one of
three ways.!
1. !Zero capacity – 0 messages
Sender must wait for receiver (rendezvous).!
2. !Bounded capacity – finite length of n messages
Sender must wait if link full.!
3. !Unbounded capacity – infinite length Sender never waits.!
Operating System Concepts!
4.42!
Silberschatz and Galvin ©1999
Exception Conditions – Error Recovery!
•  Process terminates!
•  Lost messages!
•  Scrambled Messages!
Operating System Concepts!
4.43!
Silberschatz and Galvin ©1999